The present disclosure relates to nitride semiconductor devices, and more particularly, to nitride semiconductor devices including electrode pads above an active region.
Group III-V nitride semiconductors which are represented by the general formula: AlxGa1-x-yInyN, where 0≦x≦1, 0≦y≦1, and 0≦x+y≦1, have a wide band gap and a direct transition band structure as their physical characteristics, and thus are applied to short-wavelength optical devices. Further, application of the group III-V nitride semiconductors to electronic devices is also under consideration because the semiconductors have a high breakdown electric field and high electron saturation velocity as their characteristics.
In particular, hetero-junction field effect transistors (HFETs) utilizing two-dimensional electron gas (2DEG) produced at the interface between an aluminum gallium nitride (AlxGa1-xN, where 0<x≦1) layer and a gallium nitride (GaN) layer which are sequentially formed by epitaxial growth on a semi-insulating substrate are being developed as high output devices and high frequency devices. In the HFETs, in addition to electrons which are supplied from a carrier supply layer (i.e., an n-type AlGaN Schottky layer), charges are supplied by a polarization effect caused by spontaneous polarization and piezoelectric polarization. Consequently, the HFETs made of group III-V nitride semiconductors have an electron density higher than 1013 cm 2, which is higher than that of HFETs made of aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs) by about one digit. As such, the HFETs made of group III-V nitride semiconductors are expected to have a higher drain current density than that of the HFETs made of GaAs. An element having a maximum drain current larger than 1 A/mm is reported. See, for example, Yuji Ando, Yasuhiro Okamoto, Hironobu Miyamoto, Tatsuo Nakayama, Takashi Inoue, Masaaki Kuzuhara, Evaluation of High Breakdown Voltage AlGaN/GaN Heterojunction FET, IEICE Technical Report, ED2002-214, CPM2002-105 (2002-10), pp. 29-34. Furthermore, since group III-V nitride semiconductors have a wide band gap (for example, GaN has a band gap of 3.4 eV) and also exhibit high breakdown voltage characteristics, the HFETs made of group III-V nitride semiconductors can have a breakdown voltage of 100 V or more between a gate electrode and a drain electrode. Therefore, application of electronic devices made of group III-V nitride semiconductors such as HFETs to high frequency elements and elements capable of handling greater power and smaller in size than conventional devices is under consideration. The above described characteristics of group III-V nitride semiconductors enable group III-V nitride semiconductor devices to have an active region of about one-third to one-tenth the sizes of an active region in silicon (Si) semiconductor devices. However, conventional group III-V nitride semiconductor devices, whose electrode pads for connection of wiring have large areas, have a disadvantage that the conventional group III-V nitride semiconductor devices cannot be sufficiently downsized. In particular, when the group III-V nitride semiconductor devices are used as power devices through which a large current passes, the pads can be downsized to a limited extent because it is desirable that wires and ribbons connected to the pads have large diameters and large sizes.
To overcome this disadvantage, the so-called pad-on-element structure in which electrode pads are formed above an active region is suggested in Japanese Unexamined Patent Publication No. 2008-177527, for example. When pad-on-element structure is employed in a power device which handles a high voltage, it is necessary to form an interlayer film having a large thickness in order to prevent a leakage current from being generated between the electrode pads and the electrodes located under the pads.
In order to obtain a high-efficiency device, it is essential to reduce on-resistance of the device. Further, a semiconductor device to be used as a power device needs to have characteristics of being capable of handling a large current and of having a high-breakdown voltage. Reducing on-resistance and increasing gate width allow a semiconductor device to possess these characteristics, and thereby cause the semiconductor device to have a larger maximum current.